1. Field of Invention
The present invention relates generally to modifying plants, and more particularly, to modifying expression of genes to enhance disease resistance therein.
2. Related Art
Plants have evolved a sophisticated innate immune system to defend themselves from the attack of potential pathogens. On top of this system are disease resistance (R) genes that recognize specific pathogen strains and initiate a battery of defense responses including a rapid production of reactive oxygen species (oxidative burst), induction of pathogenesis-related (PR) genes and a more readily detectable hypersensitive response (HR) (Hammond-Kosack and Jones, 1997; McDowell and Dangl, 2000; Glazebrook, 2001). HR is often manifested as rapid, localized death of plant cells at the infection site that contains the invading pathogen, and is believed to be a form of programmed cell death (PCD) analogous to animal apoptosis (Morel and Dangl, 1997; Lam et al., 2001).
A current challenge is to understand the mechanisms that link R gene-mediated pathogen recognition to the expression of resistance and the accompanying HR. At least 5 structurally distinct classes of R proteins have been characterized. The majority of isolated R genes encode members of a superfamily of nucleotide binding site and leucine-rich repeat (NB-LRR) containing proteins (Dangl and Jones, 2001). Interestingly, plant defense responses triggered by different types of R genes upon recognition of pathogens carrying corresponding avirulence (Avr) genes are often very similar (Hammond-Kosack and Jones, 1997; McDowell and Dangl, 2000; Glazebrook, 2001). This suggests that different types of R genes may activate common downstream signaling pathways that lead to the expression of resistance.
In 2001, a novel type of plant R gene RPW8 that confers broad-spectrum resistance in Arabidopsis to powdery mildew (Erysiphe) pathogens (Xiao et al., 2001) was discovered by one of the present inventors. The predicted products of the gene RPW8 of Arabidopsis are small, basic proteins with a putative N-terminal transmembrane domain and a coiled coil domain (Xiao et al., 2001). They lack the nucleotide binding site and Leu-rich repeats that characterize the products of the other Arabidopsis R genes (Dangl and Jones, 2001). RPW8 confers resistance to all tested isolates of the four species of powdery mildew pathogens of Arabidopsis (Xiao et al., 2001). By contrast, most other R genes confer resistance to only one or a few isolates of a pathogen species carrying the corresponding Avr genes (Hammond-Kosack and Jones, 1997). Despite these differences, resistance mediated by RPW8 is characterized by an HR involving the formation of H2O2. The HR triggered by the RPW8 genes involves the defense signaling components salicylic acid (SA) and EDS1 (Xiao et al., 2001). Thus, disease resistance regulated by the RPW8 genes is similar to that regulated by the other Arabidopsis R genes. Although the mechanisms by which R proteins induce HR are largely unknown, influx of calcium, protein phosphorylation and dephosphorylation, production of reactive oxygen intermediates and nitric oxide, and SA synthesis are associated with the onset of HR (Greenberg et al., 1994; Dangl et al., 1996; Lamb and Dixon, 1997; Grant et al., 2000; Glazebrook, 2001; Zhang and Klessig, 2001).
Discovery and understanding the effects of different types of R genes is important, however, modulating the activity of such genes is only possible when the interactions with other components in the plant cells are determined. Further, plant defense systems cannot be adequately activated upon pathogen attack in plants lacking cognate disease resistance (R) genes. Unfortunately, the majority of commercial crop cultivars possess fewer R genes and they are thus generally more susceptible to pathogens as compared with their wild relatives.
Utilization of R genes to protect plants against pathogens has certain limitations as well. First, plant R genes often do not function in unrelated plant genera due to “restricted taxonomic functionality” (Tai et al., 1999). Second, introduction of R genes from resistant germplasm into commercial cultivars requires time-consuming breeding programs. Third, most R genes confer resistance to only one or a few strains of a particular pathogen (Dangl and Jones, 2001). Fourth, R gene-mediated resistance is often overcome by pathogens in a short period of time.
Recent studies in this field have revealed that the signal transduction pathway(s) of plant R genes is highly conserved among different plant species (Chern et al., 2001; Liu et al., 2002). Therefore, there is a potential and a need in the art for determining interacting components that modify the activities of R genes and provide a means for enhancing pathogen resistance by controlling (e.g. up- or down-regulating) the determined key downstream components of the R gene signaling pathway(s).